An electric vehicle and a plug-in hybrid vehicle include an inverter device which drives a motor by a high voltage battery for a driving force and a low voltage battery which is used to operate auxiliaries such as lights, a radio, and the like of the vehicle. In such a vehicle, there is mounted a DC-DC converter which converts power from the high voltage battery to the low voltage battery or from the low voltage battery to the high voltage battery (for example, see PTL 1). In a housing of the DC-DC converter, a coolant channel is provided in a converter housing to cool down heated components therein, and a coolant such as a long-life coolant is supplied.
By the way, an output current required for the DC-DC converter for a vehicle reaches even 200 A. Therefore, heat amounts of the secondary winding and the secondary circuit of a transformer are large, and thus it is difficult to achieve a miniaturization and a low cost due to a countermeasure against the temperature rise. For example, as disclosed in PTL 2, there may be employed a configuration of a parallel DC-DC converter in which the secondary circuit is manufactured in a module type and provided in parallel to share the output current and to disperse the heat amount.